The present invention relates generally to an electronic circuit for measuring AC electrical energy and, more particularly, for measuring volt-amperes reactive (VAR's).
It is known to those skilled in the art to provide electronic circuits for measuring AC electrical energy. In one particular type of electronic circuit, a multiplier circuit is used to produce signals that are responsive to the product of the current and voltage of the metered circuit. A voltage-to-frequency converter can be used to provide an output signal that is responsive to the magnitude of this product.
There are various ways known to those skilled in the art for forming this signal that is representative of the product of current and voltage. U.S. Pat. No. 4,242,634, which issued on Dec. 30, 1980 to Metcalf, discloses an electronic multiplying circuit that is intended for use in an electronic watthour meter. This particular circuit comprises a variable-transconductance multiplier of the type comprising two emitter-coupled pairs of transistors with the collectors of one pair being cross-coupled with the collectors of the other pair. In order to permit the effective polarity of one of the input signals to the multiplier to be reversed from time to time for drift correction purposes, this input signal is applied to a semiconductor switching circuit which applies it alternately to the respective bases of a further emitter-coupled pair of transistors. The collectors of the transistors of this further pair are connected in series with the coupled emitters of respective ones of the emitter-coupled pairs in the variable-transconductance multiplier.
U.S. Pat. No. 4,254,376, which issued on Mar. 3, 1981 to Steinmuller, discloses an apparatus for measuring the electric power or energy in an AC network. This apparatus comprises a multiplying device that is operated according to the principle of pulse duration-pulse height modulation. Variables that are proportional to the load current and the load voltage are taken directly from the load current and the load voltage respectively.
U.S. Pat. No. 4,315,212, which issued on Feb. 9, 1982 to Gamoh, discloses an electronic watthour meter comprising a pulse width modulation circuit for subjecting a voltage signal, which is proportional to the load voltage of a power supply line, to pulse width modulation to obtain a pulse width duty cycle signal. A multiplication circuit comprises a plurality of analog switches that are selectively operated, with the aid of the pulse width duty cycle signal, so that positive and negative DC voltages that are equal in absolute value are obtained from the product of a voltage signal which is proportional to the consumption current of the power supply line and the pulse width duty cycle signal that is representative of the voltage signal proportional to the load voltage. It further comprises a dual-slope type frequency conversion circuit for converting the positive and negative DC voltages into a frequency signal.
U.S. Pat. No. 4,182,983, which issued on Jan. 8, 1980 to Heinrich et al, discloses an electronic AC electric energy measuring circuit that converts a di/dt analog input signal into a pulse width modulated signal responsive to the amplitude of a current component of an alternating current energy quantity to be measured. The pulse width modulated (PWM) signal is produced by a first modulator circuit including an integrator circuit deriving both a modulating frequency signal and a current analog signal proportional to the current component. A multiplying circuit receives a voltage analog input signal and applies it to a reference input of a second modulator signal. The modulating control of the second modulator circuit receives the pulse width modulated signal so that the multiplying circuit produces a variable amplitude and pulse width modulated signal having an average value equal to an average power measurement. An analog to frequency converter including a second integrator circuit receives the output of the multiplying circuit to produce an output pulse representative of a quantized amount of electric energy.
U.S. Pat. No 463,311, which issued on July 31, 1984 to Koboyoshi, discloses an electronic electric energy meter that comprises a delay time setting circuit that is connected to one of a voltage transformer and a current transformer that are, in turn, connected to power lines for providing a necessary delay time and a delay circuit. One embodiment of this electronic meter utilizes a shift register to provide a preselected time delay to a pulse width modulated signal. A constant frequency clock is used to control the speed of data through the shift register and various time delays are obtained by outputting the pulse width modulated signal from different stages of the shift register. To increase the time delay, the pulse width modulated signal is caused to be output from later stages of the shift register and, conversely, to decrease the time delay the pulse width modulated signal is caused to be output from earlier stages of the shift register. By properly selecting the chosen output stage of the shift register, the pulse width modulated current signal can be delayed by an output of time equivalent to 90.degree. phase shift of the signal.
U.S. patent application Ser. No. 590,110, which was filed on Mar. 9, 1984 by Mackenzie et al and assigned to the assignee of the present application, discloses a pulse width modulator for an electronic kilowatt-hour meter that includes an integrator having its inputs connected to a squarewave clock signal and a sine wave signal proportional to the time derivative of current (di/dt) on a metered circuit. It comprises a plurality of series-connected inverters that are connected to the output of the integrator to operate as a comparator which generates a pulse width modulated signal having a duty cycle proportional to metered current. It provides a negative feedback path that includes a low pass filter which is connected between the output of the last inverter and an input to the integrator to compensate for errors produced by the integrator and inverters.
U.S. Pat. No. 4,124,821, which issued on Nov. 7, 1978 to Petr, discloses an analog-to-frequency converter for developing an output signal of frequency proportional to a measuring current or voltage by the charge quantity compensation method. This type of analog- to-frequency converter is suitable for use in an electronic meter for converting an analog quantity, such as a voltage signal representing the product of voltage and current in a metered circuit, to a series of pulses whose frequency is proportional to an input signal.
U.S. patent application Ser. No. 591,139, now U.S. Pat. No. 4,596,951, which was filed on Mar. 19, 1984 by Heinrich et al and assigned to the assignee of the present application discloses an electronic circuit for a solid state electrical energy meter that has a pair of mutual inductance current transformers, a pulse width modulation circuit, an analog switch and a voltage-to-frequency converter. The pulse width modulation circuit includes an integrator that has a first operational amplifier with a feedback circuit providing a constant DC gain and a variable gain at the power frequency. The pulse width modulation circuit also includes a second operational amplifier that is configured as a comparator and connected to the first operational amplifier so that the reference level of the comparator is proportional to the average DC output of the first operational amplifier. It further comprises summing resistors that are connected to the input of the integrator and are of like value and formed in a common array.
In certain metering applications, it is desirable to measure the volt-amperes reactive (VAR's) to more properly charge the consumer for the amount of electrical energy consumed during a particular billing period. The present invention relates specifically to an electronic circuit that is usable within an electronic electric energy measuring circuit for the purposes of measuring voltamperes reactive.
The metering apparatus of the present invention comprises a current input means for providing a signal that is proportional to the time derivative of an alternating current (di/dt) in the metered circuit. This current input means can comprise an air core transformer. U.S. Pat. No. 4,368,424, which issued on Jan. 11, 1983 to Miller, discloses a transducer which is suitable for the purpose of providing a signal that is proportional to the time derivative of an alternating current. An integrating pulse width modulator (PWM) utilizes this time derivative signal, in conjunction with a clock that provides a series of periodic clock pulses, to produce an output signal that is a pulse width modulated waveform whose duty cycle is representative of the current magnitude in the metered circuit.
This pulse width modulated signal can be multiplied by a voltage signal that is representative of the magnitude of the voltage of the metered circuit. A voltage-to-frequency converter can be used to provide signal pulses representative of the magnitude of the product signal.
In the present invention, a means is provided for delaying the pulse width modulated signal, which is representative of the current magnitude of the metered circuit, by a predetermined amount of time. In a preferred embodiment of the present invention, this delaying means comprises a shift register that digitally shifts the pulse width modulated current signal. In conjunction with the shift register, the present invention utilizes a clock that controls the rate by which the data in the shift register is shifted. This clock provides a series of pulses whose frequency is determined as a function of the power line frequency. The frequency of this clock and the number of stages in the shift register are interrelated and are preselected to cause the pulse width modulated signal to be shifted by a preselected amount which, in the preferred embodiment of the present invention, represents a 90.degree. delay relative to the pulse width modulated signal that is representative of the current in the metered circuit.
The present invention also provides a voltage input means whose output is a signal that is proportional to the voltage of the metered circuit. This voltage input means can be a potential transformer whose turns ratio is suitable to reduce the metered voltage to a magnitude suitable for the other components of the electronic metering circuit. The signal from the voltage input means is multiplied by the shifted pulse width modulated signal to provide a signal that is representative of the magnitude of volt-amperes reactive (VAR's). This VAR signal is input to a voltage-to-frequency converter which produces a series of pulses representative of its input signal. In a preferred embodiment of the present invention, this frequency signal is input to a counter that accumulates its pulses to obtain a count representative of VARH's.